A lithographic apparatus applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In that instance, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. comprising part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Known lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at one time, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
In IC fabrication, the continuing improvements in microprocessor speed, memory packing density and low power consumption for micro-electronic components require a continuing decrease of size of the patterns that are transferred from the patterning device to the substrate by the lithographic apparatus. As the size of an integrated circuit is reduced and its density increases, however, the CD (critical dimension) of its corresponding patterning device pattern approaches the resolution limit of the lithographic apparatus. The resolution for a lithographic apparatus is defined as the minimum feature that the apparatus can repeatedly expose on the substrate. Various techniques, known as resolution enhancement techniques, have been applied in order to extend the resolution limit of the lithographic apparatus.
One technique to improve resolution is off-axis illumination. With this technique, the patterning device is illuminated at a selected non-perpendicular angle which may improve resolution, and particularly improves the process latitude by increasing the depth of focus and/or contrast. The angular distribution at the patterning device plane, which is an object plane, corresponds to a spatial distribution in a pupil plane of the optical arrangement of the lithographic apparatus. Typically, the shape of the spatial distribution in a pupil plane is referred to as an illumination mode. One known illumination mode is annular, in which the conventional zero order spot on the optical axis is changed to a ring-shaped intensity distribution. Another mode is multipole illumination in which several spots or beams are produced which are not on the optical axis. Examples of multipole illumination modes are dipole, comprising two poles and quadrupole, comprising four poles. For illumination modes such as dipole and quadrupole, the size of the poles in the pupil plane can be very small compared to the total surface of the pupil plane. Consequently, all of the radiation used for exposing the substrate traverses the various optical elements at or near the pupil planes at the locations of these poles only. A fraction of the radiation traversing the one or more optical elements (e.g., one or more lenses) is absorbed by the element(s). This leads to a non-uniform heating of the element(s) by the radiation beam, resulting in a local change in refractive index and a deformation of the element(s). The local change in refractive index and deformation of the element(s) results in a distorted aerial image as projected by the projection system onto the resist layer.